It's a common scenario: you've started assembling the ingredients for a cake -- the eggs have been cracked, the butter mixed in -- when you realize you have no cake flour in the house. You have plenty of all-purpose flour, but the cake flour bag is empty. Rather than turning your cake batter into a bunch of sugar cookies, with a few adjustments you'll be able to use the all-purpose flour in your cake instead.
Consider final cake texture. Substituting all-purpose flour for cake flour works better in some recipes than others. Fluffier cakes like angel food do better with cake flour because of the lower gluten content. Using all-purpose flour for pound cakes, red velvet cakes and general sheet cakes will not impact their texture.
Remove some of the flour. In order to successfully substitute all-purpose flour for cake flour, the standard formula is to remove two tablespoons of flour per cup of flour used. If your recipe calls for two cups of cake flour, remove four tablespoons of all-purpose flour. If your recipe calls for a cup and half of cake flour, measure a cup and a half of all-purpose flour and remove three tablespoons of flour before sifting. While sifting is not required for a successful substitution, it is a step in most baking recipes.
Add cornstarch. If you are concerned about your cake's texture, replace the removed flour with cornstarch. Corn starch is gluten-free and mixing it with your all-purpose flour will lower the overall gluten content. Mix the cornstarch with the flour, then sift and use as the recipe indicates.
Substitute cake flour for all-purpose flour. You can also use cake flour when your recipe calls for all-purpose flour and you don't have enough on hand. Reverse the process -- instead of removing two tablespoons of flour per cup as above, add two tablespoons of flour per cup. Whatever you're making may come out of the oven a little fluffier than usual, but should taste just fine.